A child, one of my own, asked me a simple question today: aren't worms considered insects? I didn't know the answer. But Wikipedia does.

From Wikipedia:
There are hundreds of thousands of species [of worms] that live in a wide variety of habitats other than soil. Over time this broad definition narrowed to the modern definition, although this still includes several different animal groups. Phyla that include worms include:
* Acanthocephala (spiny-headed worms)
* Annelida (segmented worms)
* Chaetognatha (arrow worms)
* Gnathostomulid (jaw worms)
* Hemichordata (acorn/tongue worms)
* Nematoda (roundworms)
* Nematomorpha (horsehair worms)
* Nemertea (ribbonworms)
* Onychophora (velvet worms)
* Phoronida (horseshoe worms)
* Platyhelminthes (flatworms)
* Priapulida (phallus worms)
* Sipuncula (peanut worms)

The most common worm is the earthworm, a member of phylum Annelida. Earthworms in general have been around for 120 million years, and are theorized to have evolved during the time of the dinosaurs. ...They lack a brain but have nerve centers (called ganglia); they also lack eyes but can sense light with photoreceptors. Worms are hermaphrodites (both sexes in one animal) but can cross fertilize.

Other invertebrate groups may be called worms, especially colloquially. In particular, many unrelated insect larvae are called "worms", such as the railroad worm, woodworm, glowworm, bloodworm, inchworm, mealworm, or silkworm.

Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms). Hence "helminthology" is the study of parasitic worms. When an animal, such as a dog, is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms.

"Ringworm" is not a worm at all, but a skin fungus.